1. Field of the Invention
The present invention relates to a filtering method and apparatus for improving resolution of an ultrasound image in an ultrasound imaging system, and more particularly, to a filtering method and apparatus for efficiently minimizing influences of signal components corresponding to lateral direction ultrasound field characteristics included in a signal of a mainlobe direction corresponding to an image point.
2. Description of the Related Art
Transmission and reception ultrasound field characteristics in an ultrasound image are determined by a transducer, an ultrasound frequency, and an ultrasound focusing system employed in an ultrasound imaging system. Here, the width of a mainlobe, the magnitude of a sidelobe, and a gratinglobe in an ultrasound field determine resolution of an image. In general, a focusing system is widely used in order to increase resolution of an ultrasound image. In case of receive focusing, in particular, dynamic receive focusing is used. As a result, maximum resolution restricted by diffraction can be obtained at all image depths. In case of transmission focusing, since the transmission focusing is possible only with respect to a single image point, resolution of an ultrasound image varies according to the depth of an image.
In order to improve resolution of the ultrasound image, there have been proposed various signal processing methods. In general, apodization is used to decrease a sidelobe, which increases however the width of a mainlobe to accordingly lower a resolution. In addition to the performance of the focusing system, attenuation, refraction and reflection in a human body lower the ultrasound field characteristics of an ultrasound image when the ultrasound image travels inside an object, that is, the human body. The change of the ultrasound field characteristics is decreased using a deconvolution filter, to thereby compensate for the ultrasound field characteristics which lower according to the depth of an image point. However, this method has a problem that the pattern of the traveling ultrasound field should be accurately understood. Recently, there have been proposed adaptive filters for compensating for a phase aberration due to nonouniformity of an ultrasound velocity, in which all factors lowering resolution in a medium are modeled by velocity non-uniformity, to thereby calculate a signal delay of each reception element and adjust a focusing delay time of a focusing system according to the calculated signal delay to improve resolution. O""Donnell et al. (U.S. Pat. No. 5,186,177) have proposed a filter considering ultrasound field characteristics by applying phase aberration correction to a human body image. By this method, lowering of ultrasound field characteristics have been compensated for by scaling an image value according to the magnitudes of all signals in any directions. This method has been applied to a human body resulting in a good result, which has however raised a problem of increasing an amount of calculation according to an increase in iteration.
To solve the above problems, it is an object of the present invention to provide a filtering method for remarkably improving resolution of an ultrasound image by efficiently minimizing an influence of lateral direction signal component which lowers resolution of the ultrasound image among ultrasound field characteristics.
It is another object of the present invention to provide an apparatus adopting the above filtering method.
To accomplish the above object of the present invention, there is provided a filtering method for improving resolution of an ultrasound image in an ultrasound imaging system, the filtering method comprising the steps of: (a) obtaining magnitude of at least one lateral direction signal component among ultrasound field characteristics which lower resolution of an image, using Fourier transform; and (b) scaling a signal focused in a mainlobe direction according to a ratio between the magnitude of the obtained lateral direction signal component and that of the signal focused in the mainlobe direction.
There is also provided a filtering apparatus for improving resolution of an ultrasound image in an ultrasound imaging system, the filtering apparatus comprising: a lateral direction signal calculator for obtaining magnitude of at least one lateral direction signal component among ultrasound field characteristics which lower resolution of an image, using Fourier transform; and a filtering unit for scaling a signal focused in a mainlobe direction according to a ratio between the magnitude of the lateral direction signal component obtained in the lateral direction signal calculator and that of the signal focused in the mainlobe direction.
Here, the signal components corresponding to the lateral direction sound field characteristics which are considered in the filtering method and apparatus are at least one of the lateral direction signal components each corresponding to a sidelobe direction, a null direction and a gratinglobe direction, on the basis of a mainlobe direction corresponding to an image point. In more detail, it is preferable that the signal component corresponding to the direction of sin(xcex8s)≈xc2x1(n+0.5)xcex/D, the signal component corresponding to a null direction is the lateral direction signal component corresponding to the direction of sin(xcex8n)=xc2x1nxcex/D and the signal component corresponding to a gratinglobe direction is the lateral direction signal component corresponding to the direction of sin(xcex8g)=xcex/d. Here, n is a positive integer, xcex is the wavelength of an ultrasound, D is the whole size of an array transducer, and d is the size of one element of the array transducer.